Sewage entering a treatment works can be described as a mixture of four pollution forms, which are rags and paper, grit from soil and roads, organic sludge and dissolved material. The rags and paper suspended in the flow are usually removed by passing the flow through mechanically raked screens. The organic sludge is removed by settlement in large sedimentation tanks in which the flow conditions are quiescent. This material has almost the same specific gravity as water and settles slowly. In turbulent flow conditions, it remains effectively in suspension in the water. The dissolvable material in the water has to be treated. The grit from soil and roads has a specific gravity considerably greater than water. There are a number of methods available for removal of the grit and it is this field of treatment of sewage to which this invention is directed.
Sewage therefore has to be treated in four phases which are usually carried out in the following sequence. Firstly, screenings removal, then grit settlement/removal, then primary sedimentation and then bio-oxidation. Final settlement or other final processes may be added. Occasionally, a further fine screening stage is introduced after the grit removal. The screenings would clog up pipes and plant if allowed to remain in the flow until the settlement stage. Grit could be left in the flow to the primary settlement stage, but may settle out of the connecting pipes and channels causing a maintenance problem. The main reason for removing as much of the grit as possible is to ease the removal and treatment of the settled sludge. The heavier material of the grit renders the sludge more solid and can lead to blockage. During treatment of the sludge, the grit often causes an action similar to concrete setting and has to be dug out of the plant. This binding and setting of grit is known as accretion.
The grit is transported in water by means of several mechanisms. If the flow is fast and turbulent, the majority of the grit will be transported in suspension. At more normal levels of flow, speed and turbulence, the grit particles bounce along the bottom of the channel often colliding with each other in an action know as saltation. At lower flow speeds, the grit forms dunes which are moved slowly along by detaching from their downstream faces in regions of turbulence and moving by saltation to the front of the next dune downstream. If the flow speed is even lower than this, then the dunes build up and the saltation transport between the dunes stops. In correctly designed sewers and channels in treatment works, a velocity of 0.8 to 1.2 meters per second is aimed at so that grit transport is either in suspension or by saltation.